Cardiovascular disease, including atherosclerosis, is a leading cause of death globally. Atherosclerosis is a slow, progressive disease for which an exact cause is unknown. It is believed however that atherosclerosis may start with damage or injury to an inner layer of an artery, with such damage possibly being caused by: high blood pressure, high cholesterol, high triglycerides (a type of fat (lipid) in the blood), smoking and other sources of tobacco, insulin resistance, obesity or diabetes, inflammation from diseases (such as arthritis, lupus or infections), and/or inflammation of unknown cause. Once the inner wall of an artery is damaged, blood cells and other substances often attack the injury site and build up in the inner lining of the artery. Over time, fatty deposits or plaques made of cholesterol and other cellular products also build up at the injury site and harden into calcified lesions, thereby narrowing the affected arteries.
One method for treating atherosclerosis and other forms of arterial lumen narrowing is percutaneous transluminal angioplasty, commonly referred to as “angioplasty” or “PTA,” or “PTCA” when performed in the coronary arteries. The objective in angioplasty is to restore adequate blood flow through the affected artery. Conventionally, angioplasty may be accomplished by inflating a dilatation balloon of a balloon or angioplasty catheter within the narrowed lumen of the artery to dilate the vessel by controlled stretching and tearing of the vessel wall and, to a certain extent, compressing soft plaque against the vessel wall. Inflation of the balloon is accomplished by supplying a pressurized fluid through an inflation lumen of the catheter which is connected to an inflation apparatus located outside of the patient's body. Similarly, applying suction to the inflation lumen collapses the balloon to its minimum dimension or low profile for initial placement of the balloon catheter within or removal from the target blood vessel.
A variety of angioplasty catheter designs and constructions are available. Typically, a dilatation balloon of an angioplasty catheter is constructed and configured to produce a nominal or preferred/rated inflated diameter at a specified inflation pressure, with standard inflation pressures being between two and twenty atmospheres (30 Psi to approximately 300 Psi). Medical dilatation balloons may be classified as being compliant, noncompliant or semi-compliant depending on the increase in diameter of the balloon under increasing inflation pressure. A compliant balloon is characterized by continued expansion as internal pressure increases. For instance, a compliant balloon will continue to radially expand beyond its preferred or rated inflated diameter in response to increasing inflation pressure. A noncompliant balloon is characterized by little growth of the balloon's preferred or rated inflated diameter over a range of customary inflation pressures, such as 3% growth in the nominal inflated diameter between 12 Atm and 18 Atm, as shown by the diameter compliance curve of the graph illustrated in FIG. 1A. A noncompliant balloon will have a maximum diameter growth percentage as low as is possible, typically not more than 5%. A semi-compliant balloon is one that provides, over a range of customary inflation pressures, a lesser degree of radial expansion beyond its nominal inflated diameter than a compliant balloon, such as 6% growth in the nominal inflated diameter between 12 Atm and 18 Atm, as shown by the diameter compliance curve of the graph illustrated in FIG. 1B. Typically, the radial expansion of a semi-compliant balloon is from 6 to 10%. The graphs of FIGS. 1A and 1B are based on the performance of known noncompliant and semi-compliant dilatation balloons having equivalent dimensions and wall thicknesses. The first non-compliant angioplasty balloon was invented by Stanley Levy and is described in U.S. Pat. No. RE32,983. The Levy patent describes a balloon reaching nominal, wrinkle-free diameter at 75-100 psi. Levy also teaches balloons having radial expansion beyond a nominal inflated diameter of less than 5% at 200 psi, or optionally less than 10% at 400 psi or 500 psi.
When used in a heavily calcified lesion a standard angioplasty balloon may not sufficiently disrupt the lesion but instead may “dog bone” as shown in FIG. 1C. “Dog bone” is a term of art meaning that a midsection of the balloon is prevented from expanding to its nominal inflated diameter by the hard calcification, whereas end sections of the balloon not constrained by the calcification inflate more than the midsection creating the so-called “dog bone” inflated shape. In such situations, balloon angioplasty with a standard dilatation balloon may not be effective in opening the stenosis, and the affected artery may still have a narrowing that requires a second angioplasty, or even a different treatment to be performed. One reason a standard balloon may “dog bone” is that it cannot exert enough radial force on a heavily calcified lesion before it exceeds its burst strength and fails.
Thus, it may be desirable during certain angioplasty procedures to utilize an angioplasty balloon with a higher rated burst pressure (RBP) than a conventional angioplasty balloon. It is known that increasing the wall thickness of a dilation balloon using the same material can achieve a relatively higher RBP than a standard balloon. However, the thicker-walled dilatation balloon thus-created can be expected to have drawbacks such as a larger deflated delivery profile and greater bending stiffness, e.g. reduced flexibility, a.k.a trackability, which in some circumstances may prohibit or impede navigation to, insertion within, and removal from a stenosed treatment site. As well due to processing of dilatation balloons that is necessary for beneficially orienting a material thereof, balloon walls of a certain thickness may develop a stress gradient between inner and outer surfaces of a “thicker walled” balloon that actually may inhibit the balloon from achieving higher burst strength. Thus there remains a need in the art for a high-pressure dilatation balloon for use in an angioplasty procedure that has a higher RBP such that it can be inflated to a sufficiently high pressure to break-up or compress even a heavily calcified lesion within a vessel. Such a balloon would beneficially have a noncompliant nature to avoid the “dog bone” effect and thereby dilate the resistant stenosis without over-stretching adjacent artery tissue. Further it would be advantageous for such a high-pressure angioplasty balloon to have a low profile and good trackability for navigation to and insertion within a calcified treatment site.